Band gap reference generators are known in the art. See, for example, Analysis and Design of Analog Integrated Circuits, Paul R. Gray and Robert G. Meyer, Third Edition, John Wiley & Sons. Band gap reference generators produce a reference voltage. The reference voltage produced is approximately equal to the band gap voltage of silicon, which is approximately 1.2 Volts. Band gap reference voltage generators are intended to generate a voltage output that is not significantly affected by power supply and temperature fluctuations.
FIG. 1 shows a known band-gap reference voltage generator 10. The bandgap reference voltage generator 10 shows two (bipolar) pnp structures 12 and 14 in first and second branches, respectively. The pnp structure 14 is actually made up of twenty-four pnp transistors in parallel, while the pnp structure 12 is made up of just one transistor. If equal currents are forced down each of these branches, there will be a larger base-emitter voltage drop across the pnp 12 in the branch where there is just one transistor. In the branch where there are twenty-four transistors in parallel there will be a much lower base-emitter voltage drop because of an increased base-emitter area. Using a well known relationship, the expected difference in the voltage drop can be determined if an equal current is passed down to pnp structures 12 and 14 of different sizes.
The circuit 10 further includes an operational amplifier 16 having a non-inverting input Inp coupled to the first branch, an inverting input Inn coupled to the second branch, and an output. The circuit 10 further includes a transistor m7 having a gate coupled to the output of the operational amplifier 16, having a drain coupled to a supply voltage, and having a source. The circuit 10 generates an output reference voltage vref, at the top of the first and second branches, coupled to the source of the transistor m7.
The aim of the circuit 10 is to find a current such that the input points Inp and Inn of the operational amplifier 16 will have equal voltages. Thus, a resistor r1 is provided in the second branch between the inverting input Inn and the transistor 14, to provide a voltage drop that compensates for the lower voltage drop of the multiple parallel transistors in the second branch that make up the transistor 14.
A band gap reference voltage generator ideally has a zero temperature coefficient. The temperature coefficient of pnp transistors, which is the change in base-emitter voltage relative to temperature, is approximately -2.2 milliVolts per degree Celsius. On the other hand, the temperature coefficient for the voltage dropped across resistor r1 is positive, and is approximately 0.270 milliVolts per degree Celsius. Therefore, it is desirable to multiply up the temperature coefficient of the voltage dropped across the r1 resistor such that the temperature coefficient is equal to that of the base-emitter voltage across a pnp transistor, to thereby provide a net zero temperature coefficient. The way this is done is by providing a resistor r2 in the second branch between the source of the transistor m7 and the inverting input. The resistor r2 is equal in resistance to some multiple of r1. For example, the resistor r2 can be made up of multiple of the resistors r1 in series, providing a resistance equal to the sum of the series resistances. Because equal currents are desired in both branches, the resistance of resistor r3 equals the resistance of resistor r2 (current equals voltage divided by resistance, and both resistors are coupled to the same voltage, vref).
In order to have temperature coefficients cancel each other, that value of r3 is equal to approximately eight times the resistance of resistor r1. The ideal voltage desired for the vref value is 1.25 Volts, which is the silicon band gap value. The characteristic curve of voltage over temperature is centered such that between about -40.degree. C. and +85.degree. C. there is little change in the vref value. Because ambient temperature varies only slightly within that range, the circuit shown in FIG. 1 is satisfactory for many purposes.
The operational amplifier 16 turns on transistor m7 until the voltages of the inputs Inn and Inp of the operational amplifier are equal, and it then holds. The voltage dropped across the resistor r1 then compensates for the larger (times twenty-four) pnp structure 14.
A problem with using the classical band gap reference voltage generator of FIG. 1 is that it includes an operational amplifier. Use of an operational amplifier involves high current consumption in the order of say hundreds of microAmps. For battery powered situations, where it is desired to use a band gap reference voltage generator, it is difficult to have minimum current consumption when an operational amplifier is being used. Further, operational amplifiers have their own temperature coefficients, so the circuit of FIG. 1 does not have the ideal temperature coefficient characteristic. Further, the operational amplifier must be well designed--voltage offset must be minimal, the current sources in the operational amplifier must matched, and there are also channel length modulation effects to counteract.
FIG. 2 shows another band gap reference voltage generator circuit 18. The circuit 18 includes a PTAT (Proportional To Absolute Temperate) current source including first and second branches 20 and 24. The branch 20 includes a pnp structure 26 made up of a single transistor, and the branch 24 includes a pnp structure 28 having thirteen pnp transistors. The circuit 18 includes a CMOS implementation of a bipolar PTAT current source.
The circuit 18 includes a p-channel transistor 30 in the first branch 20, a p-channel transistor 32 in the second branch 24, an n-channel transistor 34 in the first branch 20, and an n-channel transistor 36 in the second branch 24. These transistors have a current loop gain of one. In other words, in the stable state there are equal currents in the branches 20 and 24, and the loop gain is equal to one. The second branch 24 includes a resistor 38 coupled between the transistor 36 and the pnp structure 28 similar to the resistor r1 of FIG. 1. At power-up, the loop gain is greater than one. Current increases and increases until the voltage dropped across the resistor 28 exactly compensates for the larger pnp structure.
A positive temperature coefficient is defined by the voltage drop across the resistor 38. The branches 20 and 24 define a PTAT current source. The circuit 18 further includes a current mirror 40 which mirrors the current from the PTAT current source. The current mirror 40 has a times ten resistor ladder 41 defined by resistors 42, 44, 46, 48, 50, 52, 54, 56, 58, and 60. The temperature coefficient across this resistor ladder 41 is 0.218 milliVolts.times.10 per degree Celsius which is equal to approximately 2.2 milliVolts per degree Celsius. As mentioned above, the temperature coefficient of pnp transistors is approximately -2.2 milliVolts per degree Celsius. Therefore, the circuit 18 includes a pnp structure 62 coupled to the resistor ladder and having a temperature coefficient of -2.2 millivolts per degree Celsius. In the illustrated embodiment, the pnp structure 62 is made up of twenty-eight pnp transistors in parallel. When the pnp structure 62 is coupled to the resistor ladder, the negative temperature coefficient of the pnp transistor (-2.2 millivolts) cancels the positive temperature coefficient of the resistor ladder (2.2 millivolts) and therefore the output reference voltage Vref will effectively have a zero temperature coefficient.
An advantage of using the circuit 18 is that, to produce an output voltage exactly to 1.25 Volts (bandgap voltage), the base to emitter voltage drop across the structure 62 can be adjusted (e.g., by varying the transistor size) until the desired voltage of 1.25 Volts is achieved. This is possible because the -2.2 milliVolt temperature coefficient of the structure 62 is more or less independent of pnp transistor size.
A drawback of the circuit 18 is that the mirrored current in the unity gain circuit does not have a loop gain equal to one due to channel length modulation effects. Although current mirror loop gain should be one, it could be slightly higher than one, so the voltage drop across the resistor 38 would not have the correct value. This results in the output voltage, vref, having a value that varies from its intended value, and results in variation in the value of the output voltage, vref. Some attempts to solve this problem have involved cascading the current sources, but this results in the minimum voltage being increased significantly.
Another problem with the circuit 18 of FIG. 2 is that output current is very small, and does not have the ability to drive as much circuitry as the op-amp circuit of FIG. 1. The applications for band gap reference voltage generators of the type of FIG. 2 are very low power circuits, such as where current consumption is below 20 microAmps.